Only raw materials having a very high degree of purity are generally suitable for the manufacture of quartz glass, since impurities in the starting material considerably impair the properties of the quartz glass produced. The raw material normally used is rock crystal comprising at least 99.7% of SiO.sub.2 and imported chiefly from Brazil or Madagascar. By way of example, analysis of a Brazilian rock crystal showed the following composition:
______________________________________ COMPONENT PERCENT BY WEIGHT ______________________________________ SiO.sub.2 99.750 Al.sub.2 O.sub.3 0.024 Fe.sub.2 O.sub.3 0.020 TiO.sub.2 0.002 CaO 0.013 ______________________________________ (C. Dohler, Sprechsaal fur Keramik, Glas, Email, Silikate, 99 (1966) 960-965)
The increasing demand for quartz glass, resulting from technological development, has made it necessary to seek other raw materials which can replace rock crystal and whose degree of purity is equal to, or even greater than, that of rock crystal. Rock crystal can be replaced by, for example, silicic acids which, as is known, can be obtained from water glass solutions. However, the commercially available water glasses have too high a level of impurities, which, hitherto, could only be eliminated by complex and/or expensive measures such as electrolysis, dialysis, electrodialysis, ion exchange, extraction processes, and the like.
A process for the manufacture of two-dimensional silicic acid from water glass solutions is described in German Auslegeschrift No. 12 68 122. According to this process, a dilute water glass solution is poured into a mineral acid, preferably sulphuric acid, until the pH value of the reaction mixture is adjusted to 4.4 to 4.8. The reaction mixture is subsequently frozen and then thawed, and the insoluble silicic acid formed is separated from the solution, washed, and dried.
In the aforementioned Auslegeschrift, the silicic acid obtained by this process is designated as "pure". However, this silicic acid has the disadvantage that it still contains large quantities of impurities in the form of aluminum, iron, and titanium ions which cannot be removed merely by washing (see Table 1 below). Moreover, when this process is used, it is necessary to comply with a relatively narrow pH range and to use highly dilute water glass solutions. Due to the above-mentioned impurities, a silicic acid obtained in this manner does not appear to be very suitable as a substitute for rock crystal for the purpose of manufacturing quartz glass.